High purity ceramic abradable coatings

ABSTRACT

The invention is directed to a material and method for obtaining a ceramic abradable system for high temperature applications. High purity partially stabilized zirconia and/or hafnia base material has higher sintering resistance compared to conventional 6-9 weight percent yttria stabilized zirconia systems. The benefits of these systems are higher service lifetime and low thermal conductivity to achieve high operating temperatures. System includes a superalloy substrate, oxidation resistant bond coat and a thick ceramic abradable top coat. Total coating thickness is about 0.5-5 mm. In some applications an intermediate layer of high purity partially stabilized zirconia or a partially stabilized YSZ/MCrAlY cermet is applied over the oxidation resistant bond coat. In other applications an abradable system is applied on top of a grid. Additional benefits should be reduced blade wear at high operating conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) from U.S.Provisional Patent Application No. 60/724,268, filed on Oct. 7, 2005,which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO A “MICROFICHE APPENDIX”

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the material, manufacture and articleof abradable seals for use in gas turbine engines, and more particularlyto high purity zirconia and/or hafnia based ceramic abradable seals thatare thermal sprayed.

2. Description of the Related Art

Large gas turbine engines are widely used for aircraft propulsion andfor ground based power generation. Such large gas turbine engines aretypically of the axial type, and include a compressor section, acombustor section, and a turbine section, with the compressor sectionnormally preceded by a fan section. Each of the fan, compressor, andturbine sections comprises a plurality of disks mounted on a shaft, witha plurality of airfoil shaped blades projecting radially therefrom. Ahollow case surrounds the various engine sections. Located between thedisks and projecting inward from the case assembly which surrounds thedisks are a plurality of stationary vanes. During operation of the fan,compressor, and turbine sections, axially flowing gases alternatelycontact moving blades and the stationary vanes. In the fan andcompressor sections, air is compressed and the compressed air iscombined with fuel and burned in the combustion section to provide highpressure, high temperature gases that flow through the turbine section,where energy is extracted by causing the bladed turbine disks to rotate.A portion of this energy is used to operate the compressor section andthe fan section.

Engine efficiency depends to a significant extent upon minimizingleakage by control of the gas flow in such a manner as to maximizeinteraction between the gas stream and the moving and stationaryairfoils. A major source of inefficiency is leakage of gas around thetips of the compressor blades, between the blade tips and the enginecase. Accordingly, means to improve efficiency by reduction of leakageare increasingly important. Although a close tolerance fit may beobtained by fabricating the mating parts to a very close tolerancerange, this fabrication process is extremely costly and time consuming.Further, when the mated assembly is exposed to a high temperatureenvironment and high stress, as when in use, the coefficients ofexpansion of the mating parts may differ, thus causing the clearancespace to either increase or decrease. The latter condition would resultin a frictional contact between blades and housing, causing elevation oftemperatures and possible damage to one or both members. On the otherhand, increased clearance space would permit gas to escape between thecompressor blade and housing, thus decreasing efficiency.

One means to increase efficiency is to apply a coating of suitablematerial to the interior surface of the compressor and/or turbinehousing, to reduce leakage between the blade tips and the housing.Various coating techniques have been employed to coat the insidediameter of the compressor and/or turbine housing with an abradablecoating that can be worn away by the frictional contact of the blade, toprovide a close fitting channel in which the blade tip may travel. Thus,when subjecting the coated assembly to a high temperature and stressenvironment, the blade and the case may expand or contract withoutresulting in significant gas leakage between the blade tip and thehousing. This abradable coating technique has been employed to not onlyincrease the efficiency of the compressor/turbine, but to also provide arelatively speedy and inexpensive method for restoring excessively wornturbine engine parts to service.

Increased firing temperatures is another approach to improved engineefficiency. However, even nickel and cobalt superalloys are not capableof surviving long term operation at the firing temperatures of moderngas turbine engines that may exceed 1,400° C. in oxidizing environments.In order to provide additional protection to the metal components in thehottest areas of a gas turbine engine, it is known to coat the metalsubstrate with a layer of ceramic material to thermally insulate andchemically isolate the substrate from the hot combustion gasses. Awidely used material for this application is yttria stabilized zirconia(YSZ), with 6-9 weight percent yttria (Y₂O₃) being a common composition.

Technical benefits of 6-9 weight percent YSZ include a high thermalexpansion coefficient compared to other ceramics such as alumina.Typically the expansion coefficient mismatch is more comparable tosuperalloy based materials and oxidation resistant bond coat alloys.Other technical benefits of 6-9 weight percent YSZ include excellentthermal insulation, the fact that it is chemically inert for mostenvironments, thermally stability up to 1200° C. under isothermal orcyclic conditions, and general ease and cost effectiveness ofapplication through thermal spray technology.

Besides material properties, microstructure plays an important role inengine performance. An important consideration in the design of athermal barrier is that the coating be a ceramic material having acrystalline structure containing beneficial cracks and voids (porosity),imparting strain tolerance. If there were no cracks in the coating, thethermal barrier would not function, because the differences in thermalexpansion between the metal substrate system and the coating will causeinterfacial stresses upon thermal cycling that are greater than the bondstrength between them. By the creation of a crack network into thecoating, a stress relief mechanism is introduced that allows the coatingto survive numerous thermal cycles. Repeating crack networks aretypically imparted into the coating on varying space scales bymanipulating the thermodynamic and kinetic conditions of themanufacturing method, and different structures known to perform thecoating task have been optimized likewise. In addition to this, cracksare also formed during service, so the structure formed upon coatingmanufacture changes with time, depending on the starting material phasesin the manufactured coating and thermal conditions during service. Thereremains a need in the art for a coating material, coating materialmanufacturing method and coating manufacturing method that address thechanges in the coating microstructure during its service lifetime.

Besides microstructure characteristics of YSZ coatings, another designfactor determining coating lifetime is the sintering rate of thecoating. When the coating is cycled above half of its absolute meltingtemperature, the coating begins to sinter causing volume shrinkage. Asthe coating shrinks, the stress difference between the coating andsubstrate increases. At a certain amount of shrinkage (which variesdepending on the type of structure and thermal conditions duringservice), the stress difference exceeds the bonding strength of thecoating and it becomes detached. Decreasing the sintering rate of thethermal barrier increases the amount of time before the catastrophicshrinkage is experienced, so it can become a major design consideration.For high purity zirconia alloys, the onset of sintering commences attemperatures above 1000° C.

The thermal insulating properties of ceramic thermal barrier coatings(TCB) have been the subjects of many design improvements over the years.Known thermal barrier coating materials include the use of zirconiastabilized with both yttria and erbia (Er₂O₃) in designated amounts.Other thermal barrier coating materials include gadolinia and hafnia,preferably forming gadolinia-hafnia, and a zirconia-based material witha dysprosium oxide having the dual function of stabilizing the zirconiaand reducing the thermal conductivity of the zirconia due to phonons.

Abradable thermal barrier coatings require a highly porous coatingnetwork of, typically, between 20-35 percent porosity, which cannot beachieved by conventional flame spray techniques. Porosity is needed inorder for the turbine blades to cut grooves in the abradable coating.Previous testing of ceramic materials has shown that high porositylevels, in excess of about 35 volume percent, produce coatings prone toerosion damage. Porosity levels of less than about 20 volume percent areunsatisfactory because they cause excess blade tip wear. The materialfrom which the coating is formed must abrade relatively easily withoutwearing down the blade tips. This requires a careful balance ofmaterials in the coatings. In this environment, an abradable coatingmust also exhibit good resistance against particle erosion and otherdegradation at elevated temperatures. As known by those skilled in theart, however, these desirable characteristics have been difficult toobtain. The porosity levels achieved by conventional techniques forceramic coatings using conventional powders normally range between 5 and20 percent, and the porosity level, it has been found, is a directfunction of the powder size and spraying parameters, e.g., spray rate,spray distance and power levels of the spray gun.

Past turbine sealing structures have taken a variety of forms. Some ofthe currently favored approaches include complex plasma sprayedstructures that vary in composition from metal at one surface to ceramicat an outer surface with variations in composition, stress and porosityin between. These structures usually have a thickness on the order of 4mm and are costly because of the need to carefully control the substratetemperature and plasma spray conditions, during the deposition of manylayers, to achieve the correct abradable and durable structure. Suchthick seal structures will spall and fail if the deposition parametersare not followed closely. Likewise in service these seals with theirbuilt-in varying stresses are subject to foreign object damage. Whenfailure of thick seals does occur excess leakage results through theresulting wide gap.

With continuing efforts to improve gas turbine efficiency through use ofbetter seals and use of higher temperatures, there remains a need in theart for coating materials and coating application methods that providesimproved high-temperature properties and wear characteristics forabradable thermal barrier coatings.

SUMMARY

Accordingly, the invention is directed to a material and method forobtaining a ceramic abradable system for high temperature applications.High purity partially stabilized zirconia (ZrO₂) and/or hafnia (HfO₂)base material has a higher sintering resistance compared to conventional6-9 weight percent ytrria (Y₂O₃) stabilized zirconia systems. Thebenefits of these systems are higher service lifetime and low thermalconductivity to achieve high operating temperatures. System includes asuperalloy substrate, oxidation resistant bond coat and a thick ceramicabradable top coat. Total coating thickness may be in the range of about0.5-5.0 mm. In some applications an intermediate layer of high puritypartially stabilized zirconia or a partially stabilized YSZ/MCrAlYcermet is applied over the oxidation resistant bond coat. In otherapplications an abradable system is applied on top of a grid. Additionalbenefits should be reduced blade wear at high operating conditions. Theinvention provides desired coating structures so that the changes in thecoating microstructure over the in-service lifetime are either limitedor beneficial. Typical porosity levels may be between about 10-60 volumepercent and are achieved using a fugitive material of polymer, carbon orhollow ceramic particles. The coating may be applied using thermal spraymethods.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understandingof the invention and are incorporated in and constitute a part of thisspecification. The accompanying drawings illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention. In the figures:

FIG. 1 illustrates a typical ceramic abradable system;

FIG. 2 provides a graph of in-plane sintering resistance of ceramic whensubjected to a superalloy blade at 1400° C.;

FIG. 3 provides a phase diagram for zirconia (ZrO₂) with stabilizer; and

FIG. 4 provides a diagram showing a model for ceramic abradables.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

Embodiments of the present invention are used to increase the operationlife and temperature of ceramic seal systems using high purity zirconiabased ceramics. Modern large gas turbine engines employ axialcompressors and turbines. An axial flow stage comprises a plurality ofairfoils mounted radially at the periphery of a disk that is secured toa shaft. In the compressor section the airfoils act on the air tocompress it whereas in the turbine section the products of combustionact on the turbine blade to cause rotation. Axial flow stages aresurrounded by a case structure which serves in part to eliminate theleakage of flow around the free ends of the blades. Such leakage iswasteful and reduces efficiency. Modern gas turbine engine designershave gone to great lengths to devise useful sealing structures thatgenerally comprise an abradable coating on the case surface, usually incombination with a blade tip treatment which renders the blade tips hardand resistant to wear. As shown, for example, in FIG. 1, when in use,the blade tips cut a path into the abradable coating and providesealing.

In embodiments of the present invention the casing surface of the bladeis coated with a thermal barrier coating made of ultra-pure zirconia(ZrO₂) and/or hafnia (HfO₂). The ceramic abradable coating may beapplied directly to the surface or on an MCrAlY bonding layer appliedbetween the surface and the coating or applied to a graded or discretelayered cermet coating which is above the MCrAlY bond coat. FIG. 1 showsa cross-sectional view of a typical ceramic abradable coating system ina gas turbine engine. FIG. 1 shows a blade 120 that is affixed to ablade root 130 and disc 140. The disc 140 is driven so as to impartrotational movement to the blade 120. The blade 120 also has an abrasivetip 170 that may be made of an abrasive material, such as cubic boronnitride (cBN) embedded in a creep resistant alloy matrix. FIG. 1 alsoshows the casing segment 110 or shroud which acts to cover or house theentire gas turbine engine. An abradable seal coating 150 is providedbetween the abrasive tip 170 of the blade 120 and the casing segment110. The coating 150 may be applied to the surface of the casing 110through a variety of methods known in the art including thermal spraytechniques such as powder flame spray, electro-photeric deposition(EPD), or preferably plasma spray. The material may be supplied as apowder, solution or suspension. Powder may be in the form of a spraydried powder of the individual constituents and organic binder, spraydried powder of the combined individual constituents and organic binder,fused and crushed powder, agglomerated and sintered powder, plasmadensified material or powder from chemical solution routes. Typicalparticle sizes may vary, but are typically in the range of about 5-150microns when deposited by various thermal spray equipments, preferably15-125 microns for air plasma spray and about 30 microns or less for lowpressure plasma spray.

When applied, the coating forms a crack network and voids (porosity)that allow it to survive the stress of numerous thermal cycles. Cracksare also formed and destroyed during service, mainly during thermalcycling; so the structure formed upon coating manufacture changes withtime, depending on the starting material phases. Decreasing thesintering rate increases the amount of time before a critical fractionof cracks and pores are destroyed and eventual coating failure willoccur.

One reason for using the zirconia (ZrO₂)-based and/or hafnia(HfO₂)-based systems is that they have both a high thermal expansionrelative to other ceramics that is close to the thermal expansion of theMCrAlY bond coat and the metal substrate system it protects. Also thesematerial systems have a low thermal conductivity to give the highestlevel of thermal protection. Other ceramic systems do not possess bothof these functions. Zirconia is currently the preferred system,primarily because of its comparatively lower cost. Hafnia offersequivalent or superior property advantages over zirconia, but hastypically not been used in commercial applications due to its cost.Nonetheless, hafnia is almost always present in zirconia due to thedifficulty of their separation during mineral processing. While furtherdiscussion for convenience will address the zirconia-based system, thediscussion below is also applicable to hafnia-based systems.

To achieve optimal performance, the material purity of the startingmaterials (zirconia and/or hafnia) must be as high as possible withineconomic reason. For the present invention the zirconia and/or hafniahas impurities less than or equal to 0.15 weight percent, preferablyless than 0.10 weight percent. The reason for this is that impuritiessuch as silica and alumina and soda have a lower melting temperaturethan zirconia and/or hafnia and cause an increase in the sintering rateof the coating structure. The increased sintering rate is undesirablebecause any increase in sintering rate decreases the lifetime of thecoating. Sintering decreases the surface area of the crack networkwithin the coating, increasing the coating modulus over time untilfailure occurs. FIG. 2 shows the improvements in sintering resistancefor high purity 6-9 weight percent YSZ ceramics as compared to standard,commercially available YSZ materials. FIG. 2 shows reduced in-planeshrinkage at 1400° C. for one embodiment of the inventive material(shown as ZYGuard in the figure).

The material of the present invention contains zirconia (ZrO₂) and/orhafnia (HfO₂) having a low amount of oxide impurities. For purposes ofthe present invention, oxide impurities can be defined as materialswhich when combined with each other or with zirconia/hafnia form phaseswith melting points much lower than that of pure zirconia/hafnia,especially, but not limited to soda (Na₂O), silica (SiO₂) and alumina(Al₂O₃). In accordance with embodiments of the invention, the maximumlimits for known impurities in order to decrease sintering rate andtherefore increase service lifetime are about:

Na₂O - 0.1 weight percent SiO₂ - 0.05 weight percent Al₂O₃ - 0.01 weightpercent TiO₂ - 0.05 weight percent Fe₂O₃ - 0.05 weight percent CaO -0.05 weight percent MgO - 0.05 weight percent Na₂O - 0.01 weight percentSiO₂ - 0.01 weight percent A1₂O₃ - 0.01 weight percent TiO₂ - 0.01weight percent Fe₂O₃ - 0.01 weight percent CaO - 0.025 weight percentMgO - 0.025 weight percent.and preferably:

In one particular embodiment, when used as a coating and stabilized withyttria (Y₂O₃), the maximum limits for known impurities are:

Na₂O - 0.002 weight percent SiO₂ - 0.005 weight percent Al₂O₃ - 0.015weight percent TiO₂ - 0.002 weight percent Fe₂O₃ - 0.04 weight percentCaO - 0.02 weight percent MgO - 0.002 weight percent Cr₂O₃ - 0.001weight percent.

Pure zirconia exists in three crystal phases at different temperatures.At very high temperatures (>2370° C.) the material has a cubicstructure. At intermediate temperatures (1200 to 2372° C.) it has atetragonal structure. At low temperatures (below 1200° C.) the materialtransforms to the monoclinic structure. The transformation fromtetragonal to monoclinic is rapid and is accompanied by a 3 to 5 percentvolume increase that causes extensive stress in the material. Thus, purezirconia cannot fulfill the coating requirements for high-temperaturecycling. The resulting massive stress difference between the coating andsubstrate caused by the phase transformation is greater than the bondstrength between them, so the coating will detach.

FIG. 3 provides a standard phase diagram for stabilized zirconia showingthe general alloying trends for the zirconia stabilizers. (The diagramcan be found, for example, in Ceramic Phase Diagram, vol. Zirconia,figure Zr-157.) Zirconia can be either partially or fully stabilized.Fully stabilized zirconia has an ionic structure that is cubic at alltemperatures up to melting. Partially stabilized zirconia has an ionicstructure that is tetragonal and has a phase transformation betweentetragonal at higher temperatures and monoclinic plus cubic at lowertemperatures. The temperature at which phase transformation occursdepends on the stabilizer material, as each stabilizer causes adifferent amount of suppression of the tetragonal to monoclinic phasetransformation. At the suppressed temperatures, the rate of the phasetransformation is greatly reduced. Over a large number of temperaturecycles the phase transformation will slowly occur, gradually increasingthe amount of monoclinic phase present and setting up the massive stressconditions that determine coating lifetime.

The phase transformation in partially stabilized zirconia causeslocalized stresses that lead to the formation of micron-sizedmicro-cracks in the coating that cancel out some of the massive stresscaused by coating volume shrinkage. Thus, these two phenomena of thecoating structure—shrinking and cracking—work against each other andfinding a balance between them will maximize coating lifetime. Thismechanism implies then that the structure of the crack network of thecoating is changing with time as the phase of the ceramic materialchanges. This mechanism is required for a high temperature abradablecoating to survive thermal cycling.

Rising fuel cost and other factors continue to drive the need forimproved operational efficiency, and thus higher operating temperatures,of gas turbines. While yttria stabilized zirconia has been the materialchoice in the past—due in part to the fact that yttria is typicallyalmost two orders of magnitude less expensive than ytterbia(Yb₂O₅)—greater operational temperatures can be achieved using ytterbia.Zirconia partially stabilized by ytterbia provides a better composition.As the need for higher operating temperatures increases, a highercoating material cost will be tolerated, so ytterbia partiallystabilized zirconia may become a preferred thermal barrier coatingsystem. Given the trade-offs of cost and performance, a combination ofboth yttria and ytterbia stabilizers will have optimum performance tocost ratio. In addition to ytterbia stabilized zirconia, ceramicsdysprosium oxide stabilized zirconia ceramics have also shown potentialbenefits over standard YSZ ceramics for some thermal barrier/abradableseal applications.

Thus, in embodiments of the present invention, the stabilizer can be oneor any combination of the following oxides: yttria (Y₂O₃), ytterbia(Yb₂O₅), scandia (Sc₂O₃), lanthanide oxide and actinide oxide. Forpurposes of the present invention, these oxides (namely, any oxide fromgroup IIIB (column 3) of the periodic table of elements) can be referredto as rare earth oxides. The concentration range of stabilizers may varygenerally between about 4-20 weight percent, depending upon the ratio ofzirconia and hafnia in the material. Some suitable concentration rangesof stabilizers are about:

Y₂O₃—4-12 weight percent

Yb₂O₅—4-16 weight percent

Y₂O₃ and Yb₂O₅ and Sc₂O₃ or lanthanide oxide—4-16 weight percent; andpreferably about:

Y₂O₃—6-9 weight percent

Yb₂O₅—10-16 weight percent

Y₂O₃ and Yb₂O₅—4-16 weight percent.

Dysprosia (Dy₂O₃) may replace ytterbia as one of the stabilizing oxides.Thus, in accordance with embodiments of the invention, the stabilizercan be one or any combination of the following oxides: yttria (Y₂O₃),ytterbia (Yb₂O₅) scandia (Sc₂O₃), lanthanide oxide and actinide oxide.As with the previous-discussed stabilizer grouping, the concentrationrange of these stabilizers may vary depending upon the ratio of zirconiaand hafnia in the material. Suitable concentration ranges of stabilizersare about:

Y₂O₃—4-12 weight percent

Dy₂O₃—4-16 weight percent

Y₂O₃ and Dy₂O₃—4-16 weight percent

Y₂O₃ and Dy₂O₃ and Sc₂O₃ or lanthanide oxide—4-16 weight percent andpreferably about:

Y₂O₃—6-9 weight percent

Dy₂O₃—6-14 weight percent

Y₂O₃ and Dy₂O₃—4-16 weight percent

Other specific concentration ranges of stabilizers are provided inco-pending and commonly assigned U.S. patent application Ser. No.11/520,043, entitled “CERAMIC MATERIAL FOR HIGH TEMPERATURE SERVICE,”and U.S. patent application Ser. No. 11/520,044, entitled “OPTIMIZEDHIGH TEMPERATURE THERMAL BARRIER,” and U.S. patent application Ser. No.11/520,042, entitled “OPTIMIZED HIGH PURITY COATING FOR A HIGHTEMPERATURE THERMAL CYCLING APPLICATIONS” each filed on Sep. 13, 2006and each incorporated herein by reference.

By adding porosity to the coating a decrease in sintering rate can beachieved. Porosity can be introduced into the coating by adding to thematerial a different placeholder material (or fugitive phase) that canbe removed later. If the coating is manufactured from a slurry, afoaming agent can be added in addition to agitation to introduce gasbubbles which become porosity upon drying of the coating.

Typically for thermal spray applications, a polymer or organic materialin powder form can be added to the material blend. Powder may be in theform of a spray dried powder of the individual constituents and anorganic binder, spray dried powder of the combined individualconstituents and an organic binder, fused and crushed powder,agglomerated and sintered powder, plasma densified material or powderfrom chemical solution routes. The organic binder may be used to atleast partially bond together the placeholder material and the ceramicmaterial. For high temperature abradable coatings, the benefit of addinga fugitive phase is that a higher porosity than is achievable withconventional deposition methods. The increased porosity aidsabradability by introducing more surfaces to the crack network of thecoating, decreasing the coating elastic modulus and thus promotingcontrolled material removal. Thus, according to an embodiment of theinvention, a coating material may have about 70 to 99 weight percent ofa partially stabilized ultra-pure ceramic material as previouslydescribed and about 1-30 weight percent (and preferably 2-15 weightpercent) of a placeholder material. The placeholder material may be anorganic powder material or an inorganic powder material that can beburned out subsequent to deposition of the coating material.

In some typical thermal spray applications embodiments of the coatingstructure of the present invention may be formed by blending,agglomerating or cladding together the ceramic material and theplaceholder material prior to said spraying. Other thermal sprayapplications may apply the ceramic material and the placeholder materialin a co-spraying process. FIG. 4 is a typical model on how ceramicinteracts and performs as an abradable when subjected to superalloyblade. As a blade 120 rotates, the blade 120 (which may have an abrasivetip, not shown) may contact an abradable seal coating 150 made fromceramic material is accordance with the present invention. The coating150 is provided between the tip of the blade 120 and a casing segment(not shown). As the blade 120 contacts the coating 150, individualcoating particles 152 may be broken off.

In accordance with embodiments of the invention, a high temperatureabradable seal coating can be further optimized by building the coatingin layers with different compositions, or by introducing a compositionalgradient through the thickness of the coating. The reason for this isthat due to the relatively low thermal conductivity of the coating, atemperature gradient exists in the coating during high temperatureservice. Thus there is a variation in the sintering rate through thethickness of the coating, and this means that the coating structure canbe designed using the most optimal material at the surface, with lessoptimal materials towards the interface. Compositional gradients can beintroduced into the coating during manufacture by using multiplefeedstocks of different composition and varying their feed-rates duringdeposition. A few examples of discrete or graded layers could be: 1)MCrAlY bond coat, high purity YSZ intermediate coating and porous YSZtop coat; 2) MCrAlY bond coat, high purity dense YSZ intermediate layerand porous high purity ytterbia stabilized zirconia and 3) MCrAlY bondcoat, low K TBC type material using two or more multiple stabilizersfrom ytterbia, ceria, gadolinium oxide, yttria, dysprosia that arealloyed with high purity zirconia or hafnia and a thick porous ceramicseal (reduced sintering resistance) of high purity ytterbia stabilizedzirconia.

A main property affecting the abradability of the coating is the elasticmodulus. Since the elastic modulus increases with sintering, it is vitalthat a high temperature abradable coating has optimized sinteringresistance. The thermal conductivity of the abradable coating does notneed to be maximized because sometimes a higher thermal conductivity isrequired to remove frictional heating and avoid localized coatingfailure. A preferred coating structure for a high temperature abradableseal coating is lamellar with regular repeating porosity. For example,in some embodiments of the invention, properties of the high temperatureabradable seal coating can be improved by spraying the ceramic materialand the placeholder material onto a metal substrate using a thermalspray process at pressures between 1 Pa and 1 MPa, so as to form astream of molten and/or semi-molten droplets that build up a coating offrozen lamellar splats subsequent to impact with the substrate. In acoating made from this application process, high temperature shrinkageor sintering occurs less in the in-plane direction of the droplets thanin the through thickness direction. Typical porosity and micro cracksmay be in the range of about 10 to 60 volume percent, preferably in therange of about 15 to 40 volume percent.

A high purity abradable seal top coat in accordance with embodiments ofthe present invention has improved sintering resistance compared tostandard yttria stabilized zirconia based materials. Typical coatingthicknesses for the abradable coating materials described herein areabout 0.5 mm to 5 mm. In embodiments of the invention, the coatingmaterial may be applied over 1) an oxidation and/or hot corrosionresistant MCrAlY or MAl or MCr bond coat; 2) an intermediate, about 0.2to 1.0 mm thickness, zirconia-stabilized ceramic or hafnia-stabilizedceramic that has a main function of reducing heat transfer to thesubstrate and improving adhesion; or 3) an intermediate cermet of anoxidation resistant MCrAlY and a high purity zirconia/hafnia ceramicthat has a function of reducing heat transfer and minimizing thermalexpansion differences between the ceramic abradable top coat and MCrAlYbond coat or substrate. The abradable seal base material may be asuperalloy grid surface based on investment casting or some othermanufacture techniques.

While exemplary embodiments of the invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousinsubstantial variations, changes, and substitutions will now beapparent to those skilled in the art without departing from the scope ofthe invention disclosed herein by the Applicants. Accordingly, it isintended that the invention be limited only by the spirit and scope ofthe claims, as they will be allowed.

1. A high-purity coating that is suitable for high temperature cyclingapplications, said coating formed by the process comprising: providing aceramic material consisting essentially of about 4 to 20 weight percentof a stabilizer of one or more rare earth oxides, and a balance of atleast one of zirconia (ZrO₂), hafnia (HfO₂) and combinations thereof,wherein the zirconia (ZrO₂) and/or hafnia (HfO₂) is partially stabilizedby the stabilizer, and wherein the total amount of impurities is lessthan or equal to 0.15 weight percent; providing a removable placeholdermaterial comprising an inorganic powder material or an organic powdermaterial that can be burned out subsequent to deposition; and sprayingsaid ceramic material and said removable placeholder material onto ametal substrate using a thermal spray process at pressures between 1 Paand 1 MPa, so as to form a stream of molten and/or semi-molten dropletsthat build up a coating of frozen lamellar splats subsequent to impactwith the substrate, wherein the coating sprayed on the metal substrateincludes the ceramic material and the removable placeholder material,and wherein the coating sprayed on the metal substrate furthercomprises, after removal of the removable placeholder material, astructure including a ceramic matrix, porosity, and micro cracks.
 2. Thehigh-purity coating of claim 1, wherein the stabilizer is about 4-16weight percent, said stabilizer having at least one of yttria (Y₂O₃),ytterbia (Yb₂O₅), and combinations thereof.
 3. The high-purity coatingof claim 1, wherein the stabilizer is about 4-16 weight percent, saidstabilizer having at least one of yttria (Y₂O₃), dysprosia (Dy₂O₃), andcombinations thereof.
 4. The high-purity coating of claim 1, wherein thestabilizer is about 6-9 weight percent yttria (Y₂O₃).
 5. The high-puritycoating of claim 1, wherein the stabilizer is about 8-12 weight percentdysprosia (Dy₂O₃).
 6. The high-purity coating of claim 1, wherein thespraying of the ceramic material and the removable placeholder materialis a co-spraying process.
 7. The high-purity coating of claim 1, whereinthe porosity and micro cracks are in the range of about 10 to 60 volumepercent.
 8. The high-purity coating of claim 1, wherein the porosity andmicro cracks are in the range of about 15 to 40 volume percent.
 9. Thehigh-purity coating of claim 1, wherein the process of forming thecoating further comprises blending together the ceramic material and theremovable placeholder material prior to said spraying.
 10. Thehigh-purity coating of claim 1, wherein the process of forming thecoating further comprises agglomerating the ceramic material and theremovable placeholder material prior to said spraying.
 11. Thehigh-purity coating of claim 1, wherein the process of forming thecoating comprises adding an organic binder, wherein one of the ceramicmaterial and the removable placeholder material are bonded to one of theplaceholder material and the ceramic material at least partially by theorganic binder.
 12. The high-purity coating of claim 1, wherein thetotal amount of impurities of the ceramic material is less than or equalto 0.10 weight percent.
 13. A high-purity coating that is suitable forhigh temperature cycling applications, said coating comprising: aceramic material consisting essentially of about 4 to 20 weight percentof a stabilizer of one or more rare earth oxides, and a balance of atleast one of zirconia (ZrO₂), hafnia (HfO₂) and combinations thereof,wherein the zirconia (ZrO₂) and/or hafnia (HfO₂) is partially stabilizedby the stabilizer, and wherein the total amount of impurities is lessthan or equal to 0.15 weight percent; a removable placeholder materialcomprising an inorganic powder material or an organic powder materialthat can be burned out subsequent to deposition; and said ceramicmaterial and said removable placeholder material being sprayable onto ametal substrate using a thermal spray process at pressures between 1 Paand 1 MPa, so as to form a stream of molten and/or semi-molten dropletsthat build up a coating of frozen lamellar splats subsequent to impactwith the substrate, wherein the coating sprayed on the metal substrateincludes the ceramic material and the removable placeholder material,and wherein the coating sprayed on the metal substrate furthercomprises, after removal of the removable placeholder material, astructure including a ceramic matrix, porosity, and micro cracks. 14.The high-purity coating of claim 13, wherein the stabilizer is about4-16 weight percent, said stabilizer having at least one of yttria(Y₂O₃), ytterbia (Yb₂O₅), and combinations thereof.
 15. The high-puritycoating of claim 13, wherein the stabilizer is about 4-16 weightpercent, said stabilizer having at least one of yttria (Y₂O₃), dysprosia(Dy₂O₃), and combinations thereof.
 16. The high-purity coating of claim13, wherein the stabilizer is about 6-9 weight percent yttria (Y₂O₃).17. The high-purity coating of claim 13, wherein the stabilizer is about8-12 weight percent dysprosia (Dy₂O₃).
 18. A high-purity coating that issuitable for high temperature cycling applications, said coatingcomprising: a ceramic material consisting essentially of about 4 to 20weight percent of a stabilizer of one or more rare earth oxides, and abalance of at least one of zirconia (ZrO₂), hafnia (HfO₂) andcombinations thereof, wherein the zirconia (ZrO₂) and/or hafnia (HfO₂)is partially stabilized by the stabilizer, and wherein the total amountof impurities is less than or equal to 0.15 weight percent; a removableplaceholder material comprising an inorganic powder material or anorganic powder material that can be burned out subsequent to deposition;and said ceramic material and said removable placeholder material beingapplied to a metal substrate as a stream of molten and/or semi-moltendroplets that build up a coating of frozen lamellar splats subsequent toimpact with the substrate, wherein the coating on the metal substrateincludes the ceramic material and the removable placeholder material,and wherein the coating on the metal substrate further comprises, afterremoval of the removable placeholder material, a structure including aceramic matrix, porosity, and micro cracks.
 19. The high-purity coatingof claim 18, wherein the stabilizer is about 4-16 weight percent, saidstabilizer having at least one of yttria (Y₂O₃), ytterbia (Yb₂O₅), andcombinations thereof and the coating is sprayable on the metal substrateusing a thermal spray process at pressures between 1 Pa and 1 MPa.
 20. Aprocess of forming a high-purity coating that is suitable for hightemperature cycling applications, said process comprising: providing aceramic material consisting essentially of about 4 to 20 weight percentof a stabilizer of one or more rare earth oxides, and a balance of atleast one of zirconia (ZrO₂), hafnia (HfO₂) and combinations thereof,wherein the zirconia (ZrO₂) and/or hafnia (HfO₂) is partially stabilizedby the stabilizer, and wherein the total amount of impurities is lessthan or equal to 0.15 weight percent; providing a removable placeholdermaterial comprising an inorganic powder material or an organic powdermaterial that can be burned out subsequent to deposition; and sprayingsaid ceramic material and said removable placeholder material onto ametal substrate using a thermal spray process at pressures between 1 Paand 1 MPa, so as to form a stream of molten and/or semi-molten dropletsthat build up a coating of frozen lamellar splats subsequent to impactwith the substrate, wherein the coating sprayed on the metal substrateincludes the ceramic material and the removable placeholder material,and wherein the coating sprayed on the metal substrate furthercomprises, after removal of the removable placeholder material, astructure including a ceramic matrix, porosity, and micro cracks.